1. Field of the Invention
The present invention relates to a wireless communication device for performing wireless communication.
2. Description of the Related Art
A mobile phone terminal comprises, as its main components, an RF transceiver (RF-IC) for converting an RF (Radio Frequency) signal to a baseband signal and vice versa, and a baseband processor IC (PHY: physics) for processing the baseband signals. In recent years, the analog interface between the RF-IC and the PHY is being replaced by a digital interface.
As such digital interfaces, DigRF has been standardized for 3G (3rd Generation) applications and JC-61 has been standardized for WiMAX (Worldwide Interoperability for Microwave Access) applications.
These standards permit interconnection between the RF-IC and the PHY, and since hardware connectivity is guaranteed for ICs complying with the standards, the standardization is expected to accelerate liberalization of the digital mobile phone component market.
FIG. 14 illustrates a schematic configuration of a mobile phone terminal. The mobile phone terminal 5 comprises an antenna section 50, an RF-IC 60, and a PHY 70. The illustrated terminal is of a direct conversion type wherein an RF signal with a frequency equal to that of the carrier wave is directly converted to a baseband signal and vice verse without involving an IF (Intermediate Frequency) stage.
The antenna section 50 includes antennas 51 and 52, a switch 53, and band-pass filters 54a, 55b-1 and 55b-2. The RF-IC 60 includes amplifiers 61a, 61b-1 and 61b-2, an adder 62, mixers 63a-1, 63a-2 and 63b-1 to 63b-4, D/A converters 64a-1 and 64a-2, A/D converters 64b-1 to 64b-4, a multiplexer (MUX) 65, a demultiplexer (DEMUX) 66, a driver 67, a receiver 68, and a synthesizer 69. The PHY 70 includes a driver 71, a receiver 72, a multiplexer 73, a demultiplexer 74, and a logic circuit 75.
When the RF signal is to be received, the switch 53 switches to a receive side (the antenna 51 is connected to the input terminal of the band-pass filter 55b-1). The RF signal received by the antennas 51 and 52 is input to the band-pass filters 55b-1 and 55b-2 through which a desired RF frequency is passed to be sent to the RF-IC 60.
The RF signal output from the band-pass filter 55b-1 is applied to the amplifier 61b-1 in the RF-IC 60, and the amplifier 61b-1 provides differential outputs, which then are supplied to the mixers 63b-1 and 63b-2. Similarly, the RF signal output from the band-pass filter 55b-2 is applied to the amplifier 61b-2 in the RF-IC 60, and the amplifier 61b-2 provides differential outputs, which are input to the mixers 63b-3 and 63b-4.
The synthesizer 69 outputs in-phase (I) local signals I(+) and I(−) having the same frequency as the RF frequency (I(+) and I(−) are opposite in phase), and also outputs quadrature (Q) local signals Q(+) and Q(−) (Q(+) and Q(−) are opposite in phase) with the same frequency.
The mixer 63b-1 mixes the RF signal (+), which is one of the signals output from the amplifier 61b-1, with the local signal I(+) output from the synthesizer 69, to generate a baseband signal. Also, the mixer 63b-1 mixes the RF signal (−), which is the other signal output from the amplifier 61b-1, with the local signal I(−) output from the synthesizer 69, to generate a baseband signal.
On the other hand, the mixer 63b-2 mixes the RF signal (+) output from the amplifier 61b-1 with the local signal Q(+) output from the synthesizer 69, to generate a baseband signal. Also, the mixer 63b-2 mixes the RF signal (−) output from the amplifier 61b-1 with the local signal Q(−) output from the synthesizer 69, to generate a baseband signal. The mixers 63b-3 and 63b-4 carry out identical processes.
The analog baseband signals output from the mixers 63b-1 to 63b-4 are input to the A/D converters 64b-1 to 64b-4, respectively, which then perform analog/digital conversion to obtain digital baseband signals.
The A/D converter 64b-1 outputs an I digital baseband signal generated from the RF signal received by the antenna 51, and the A/D converter 64b-2 outputs a Q digital baseband signal generated from the RF signal received by the antenna 51.
Also, the A/D converter 64b-3 outputs an I digital baseband signal generated from the RF signal received by the antenna 52, and the A/D converter 64b-4 outputs a Q digital baseband signal generated from the RF signal received by the antenna 52.
The multiplexer 65 multiplexes the two I baseband signals output from the A/D converters 64b-1 and 64b-3 into one baseband signal, and also multiplexes the two Q baseband signals output from the A/D converters 64b-2 and 64b-4 into one baseband signal. The two, I and Q digital baseband signals thus obtained are output to the driver 67, which then sends the signals to the PHY 70.
In the PHY 70, the receiver 72 receives the digital baseband signals and outputs the received signals to the demultiplexer 74. The demultiplexer 74 demultiplexes the baseband signals into a number of signals equal to that before the multiplexing, which signals are sent to the logic circuit 75, where a predetermined process is performed on the received baseband signals.
A wireless technology using multiple antennas to transmit and receive data is called MIMO (Multiple Input Multiple Output). In the mobile phone terminal 5, the two antennas 51 and 52 are used to receive data, and the received data is multiplexed and then demultiplexed, thereby enabling communication tolerant to multipath fading.
When transmitting an RF signal, on the other hand, the switch 53 switches to a transmit side (the antenna 51 is connected to the output terminal of the band-pass filter 54a). The logic circuit 75 in the PHY 70 generates and outputs digital baseband signals, and the multiplexer 73 multiplexes the received baseband signals. The driver 71 sends the multiplexed baseband signals to the RF-IC 60.
In the RF-IC 60, the receiver 68 receives the multiplexed digital baseband signals and outputs the received signals to the demultiplexer 66. The demultiplexer 66 demultiplexes the baseband signals into the number of signals equal to that before the multiplexing, and outputs the demultiplexed signals to the D/A converters 64a-1 and 64a-2.
The D/A converters 64a-1 and 64a-2 each subject the corresponding baseband signal to digital/analog conversion, to generate two analog baseband signals. The mixer 63a-1 mixes one of the baseband signals output from the D/A converter 64a-1 with the local signal I(+) output from the synthesizer 69, to generate an RF signal. Also, the mixer 63a-1 mixes the other baseband signal output from the D/A converter 64a-1 with the local signal I(−) output from the synthesizer 69, to generate an RF signal.
On the other hand, the mixer 63a-2 mixes one of the baseband signals output from the D/A converter 64a-2 with the local signal Q(+) output from the synthesizer 69, to generate an RF signal. Also, the mixer 63a-2 mixes the other baseband signal output from the D/A converter 64a-2 with the local signal Q(−) output from the synthesizer 69, to generate an RF signal.
The adder 62 adds together the two signals output from the mixer 63a-1 and respectively mixed with the local signals I(+) and I(−), and also adds together the two signals output from the mixer 63a-2 and respectively mixed with the local signals Q(+) and Q(−), to generate I and Q RF signals.
The amplifier 61a combines differential inputs, namely, the two RF signals, into one signal, which is sent to the band-pass filter 54a. The band-pass filter 54a passes a desired RF frequency therethrough, the resulting RF signal being transmitted from the antenna 51 into the air.
As conventional techniques, a technique has been proposed wherein the interface between the radio portion and the baseband portion employs 8B/10B encoding for communication (PCT-based Unexamined Japanese Patent Publication No. 2006-502679 (paragraph nos. [0008] to [0013], FIG. 1)).
The range of bit rates that can be set for the digital interfaces between the RF-IC 60 and the PHY 70 of the mobile phone terminal 5 has its lower and upper limits determined, respectively, by the transmission band and the specifications of high-speed I/O devices (drivers, receivers, etc.). For example, where the RF signal received by the two branches (two antennas 51 and 52) has a frequency bandwidth of 20 MHz, the bandwidth of each of the two branch outputs after the mixing is about 10 MHz.
Also, provided the resolution of the digital signal after the A/D conversion is 10 bits (in FIG. 14, each signal line labeled “10 bits” represents ten 1-bit signal lines), the bandwidth of one A/D output line equals a bit rate of about 20 Mb/s. Accordingly, the lower-limit bit rate of the digital interface between the driver 67 and the receiver 72 is derived as 800 Mb/s (=20 Mb/s×10 bits×2 (two I and Q signals)×2 (two branches)).
The bit rate 800 MB/s is, however, a value reckoned taking account only of the I and Q signals; in practice, a minimum of 1 Gb/s is required where control signals, such as those for error correction by redundant encoding, and other header information are taken into consideration (as for the upper-limit bit rate, a maximum of about 3 Gb/s is currently available, depending on the specifications of high-speed I/O devices).
Conventional lower-speed interfaces between the RF-IC and the PHY used to have a bit rate of about 100 Mb/s. The RF frequency at the antenna section is of the order of several GHz (e.g., 1 GHz) and its frequency band is spaced significantly from that of the interface. Accordingly, conventional terminals are not associated with the problem that the transmission quality degrades due to noticeable interference.
In the latest mobile phone terminal 5, on the other hand, the digital interface between the RF-IC 60 and the PHY 70 has an even higher bit rate because of multiplexing and increased rate per signal line, as stated above.
Consequently, the frequency band of the signal transferred through the digital interface approaches the RF frequency band at the antenna section 50, and thus a problem arises in that noise produced at the digital interface leaks into the antenna section 50, causing such an interference as to degrade the transmission quality.
FIG. 15 is a conceptual diagram illustrating interference between the digital interface and the antenna section 50. The figure shows the manner of how noise produced at the digital interface leaks to the antenna section 50 through a certain isolation and enters the RF-IC 60 via an input terminal Pin for the RF signal.
Noise is thought to reach the antenna section 50 mainly through the air, GND (ground) or power supply, and electromagnetic waves generated at the digital interface travel through the air, GND or power supply to the antenna section 50. If such a phenomenon occurs, the noise affects the RF frequency and the noise-containing RF signal enters the RF-IC 60 and is processed, causing degradation of the transmission quality.
The term “isolation” represents an element that causes a change in the amount of interference (the amount of noise leak) and is an index indicating to what extent the interference is reduced. For example, where the signal output from the output terminal of a certain circuit fluctuates by 1 V and if the isolation between the input and output terminals of the circuit is −60 dB, the signal input to the input terminal fluctuates by 1 mV due to the interference. (When converted to antilogarithm, −60 dB equals 10−60/20=0.001, and therefore, a fluctuation corresponding to 1/1000 of 1 V appears at the input terminal. Namely, 1 V×0.001=1 mV.)
Similarly, if the isolation is −80 dB, the signal input to the input terminal fluctuates by 0.1 mV. (When converted to antilogarithm, −80 dB equals 10−80/20=0.0001, and therefore, 1 V×0.0001=0.1 mV.) If the isolation is −100 dB, the signal input to the input terminal fluctuates by 0.01 mV. (When converted to antilogarithm, −100 dB equals 10−100/200=0.00001, and therefore, 1 V×0.00001=0.01 mV.)
Interference occurs to a certain extent, and the amount of interference varies also in accordance with the value of the isolation as mentioned above. (Under the same environmental conditions, the smaller the value of the isolation, the smaller the amount of interference becomes.)
FIG. 16 represents frequency spectra of signals traveling through the digital interface, wherein the vertical axis indicates signal strength (mVrms) and the horizontal axis indicates frequency (MHz). The figure represents simulation results obtained with NRZ (non-return to zero) signals transferred through the digital interface between the driver 67 and the receiver 72.
Spectrum g1 represents the strength of a signal traveling through the digital interface at a bit rate of 1 Gb/s, and spectrum g2 represents the strength of a signal traveling through the digital interface at a bit rate of 1.5 Gb/s.
Where the isolation between the digital interface and the input terminal Pin is −120 dB, the strength of the signal at the digital interface is, in the case of the spectrum g1, equal to 8.6 mVrms at the RF frequency 1.5 GHz, and therefore, noise leaking from the digital interface into the input terminal Pin is equal to 8.6 nVrms (=8.6 mVrms×0.000001).
Usually, the amplitude of the RF signal at the input terminal Pin has a small value of about 1 nV, and when the bit rate of the digital interface is 1 Gb/s as in the spectrum g1, the noise has an amplitude greater than that of the signal (1 nV<8.6 nV). Consequently, the isolation value −120 dB is not small enough, and the isolation needs to be lowered further.
To set the isolation to a desired value, design and implementation may be modified so that the amount of interference may fall within an allowable range (e.g., a multilayer substrate is used to increase the thickness of the GND pattern). This technique, however, leads to increase in cost.
On the other hand, when the bit rate of the digital interface is 1.5 Gb/s as in the spectrum g2, the signal strength at the digital interface is equal to 0.2 mVrms at the RF frequency 1.5 GHz, and therefore, noise leaking from the digital interface into the input terminal Pin is equal to 0.2 nVrms (=0.2 mVrms×0.000001). In this case, the amplitude of the signal is greater than that of the noise (0.2 nV<1 nV).
Accordingly, where the isolation is −120 dB, the amount of noise leak can be reduced by raising the bit rate of the digital interface to 1.5 Gb/s. This holds true, however, only with respect to the RF frequency 1.5 GHz. The RF frequency varies depending on the wireless communication standard and the country where the terminal is used. In practice, therefore, a specific frequency cannot be set, and it is not possible to restrict the amount of noise leak to a fixed level or lower with respect to various RF frequencies.
For example, in FIG. 16, the relationship of signal strength between the spectra g1 and g2 is reversed at the RF frequency 2 GHz, compared with the relationship at the RF frequency 1.5 GHz. This reveals that for the same isolation (=−120 dB) and at the RF frequency 2 GHz, the amount of noise leak is greater when the bit rate of the digital interface is 1.5 Gb/s (spectrum g2) than when the bit rate of the digital interface is 1 Gb/s (spectrum g1).
In the case of lower RF frequencies represented in FIG. 16, the signal strength at the digital interface is high irrespective of at what bit rate the NRZ signal may be transferred, under the condition that the bit rate of the digital interface is at or above 1 Gb/s, for example. With respect to low RF frequencies, therefore, significant noise is produced at all times.
Thus, with increase in the bit rate of the digital interface between the RF-IC and the PHY, the extent to which the digital interface interferes with the antenna section 50 increases. Since the amount of noise leak varies depending on the RF frequency, however, the interference cannot be effectively restrained by merely setting the bit rate of the digital interface to a specific value.
Future RF-ICs are required to deal with various RF frequencies on a single chip, and thus it is difficult to restrain noise over the entire RF frequency range with the bit rate between the RF-IC and the PHY fixed.